In gas fired industrial furnaces, NOx is formed by the oxidation of nitrogen drawn into the burner with the combustion air stream. The formation of NOx is widely believed to occur primarily in regions of the flame where there exist both high temperatures and an abundance of oxygen. Since ethylene furnaces are amongst the highest temperature furnaces used in the hydrocarbon processing industry, the natural tendency of burners in these furnaces is to produce high levels of NOx emissions.
The majority of recent low NOx burners for gas-fired industrial furnaces are based on the use of multiple fuel jets in a single burner. Such burners may employ fuel staging, flue-gas recirculation (“FGR”), or a combination of both. U.S. Pat. Nos. 5,098,282 and 6,007,325 disclose burners using a combination of fuel staging and flue-gas recirculation. Certain burners may have as many as 8-12 fuel nozzles in a single burner. The large number of fuel nozzles require the use of very small diameter nozzles. In addition, the fuel nozzles of such burners are generally exposed to the high temperature flue-gas in the firebox.
One technique for reducing NOx that has become widely accepted in industry is known as staging. With staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NOx formation than an air-fuel fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature dramatically reduce NOx emissions. However this is generally balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase.
In the context of premix burners, the term primary air refers to the air premixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air is more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
U.S. Pat. No. 4,629,413 discloses a low NOx premix burner and discusses the advantages of premix burners and methods to reduce NOx emissions. The premix burner of U.S. Pat. No. 4,629,413 lowers NO emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The manner in which the burner disclosed achieves light off at start-up and its impact on NOx emissions is not addressed. The contents of U.S. Pat. No. 4,629,413 are incorporated by reference in their entirety.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NO emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Airflow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NO emissions. The flue-gas recirculating system may be retrofitted into existing burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
A drawback of the system of U.S. Pat. No. 5,092,761 is that the staged-air used to cool the FGR duct first enters the furnace firebox, traverse a short distance across the floor and then enter the FGR duct. During this passage, the staged air is exposed to radiation from the hot flue-gas in the firebox. Analyses of experimental data from burner tests suggest that the staged-air may be as hot as 700° F. when it enters the FGR duct.
From the standpoint of NOx production, another drawback associated with the burner of U.S. Pat. No. 5,092,761 relates to the configuration of the lighting chamber, necessary for achieving burner light off. The design of this lighting chamber, while effective for achieving light off, has been found to be a localized source of high NOx production during operation. Other burner designs possess a similar potential for localized high NOx production, since similar configurations are known to exist for other burner designs, some of which have been described hereinabove.
Additionally, commercial experience and modeling have shown when flue-gas recirculation rates are raised, there is a tendency of the flame to be drawn into the FGR duct. Often, it is this phenomenon that constrains the amount of flue-gas recirculation. When the flame enters directly into the flue-gas recirculation duct, the temperature of the burner venturi tends to rise, which raises flame speed and causes the recirculated flue gas to be less effective in reducing NOx. From an operability perspective, the flue-gas recirculation rate needs to be lowered to keep the flame out of the FGR duct to preserve the life of the metallic FGR duct.
U.S. Pat. No. 6,877,980 discloses a burner for use in furnaces such as those used in steam cracking with increased FGR recirculation rate and low NOx formation. The burner includes a primary air chamber; a burner tube having an upstream end, a downstream end and a venturi intermediate said upstream and downstream ends, said venturi including a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3; a burner tip mounted on the downstream end of said burner tube adjacent a first floor burner opening in the furnace, so that combustion of the fuel takes place downstream of said burner tip; and a fuel orifice located adjacent the upstream end of said burner tube, for introducing fuel into said burner tube. In the burner disclosed therein, a circular barrier wall is erected surrounding the floor burner opening, blocking the base of the floor burner flame from the flue-gas recirculation duct ports on the floor. The barrier wall serves the purpose of stabilizing the flame and reducing NOx formation.
It has been recently found that, however, the annular barrier wall in the burner of U.S. Pat. No. 6,877,980 also reflects the heat produced by the flame to the burner tip, thereby increasing the burner tip temperature. Where the fuel gas comprises primarily hydrocarbons such as methane, the burner tip temperature is generally reasonably low to provide a satisfactory life, even with the reflected heat from the barrier wall. However, where the fuel gas comprises primarily hydrogen (i.e., comprising at least 50 mol % of hydrogen), the flame speed and flame temperature are significantly higher, and so is the amount of heat reflected by the barrier wall to the burner tip. As a result, the burner tip is frequently overheated to an exceedingly high temperature, leading to premature failure, especially during burner turn-down process or flame flash-back.
Therefore, there is a need for an improved burner sub-system design with reduced overheating potential, especially when hydrogen-rich fuel gas is used. The present invention satisfies this and other needs.